This invention relates to a plasma processor which is a semiconductor fabrication apparatus, and more particularly to a plasma processor which generates a plasma by the use of electron cyclotron resonance and which can effect a uniform plasma process over a large area of a substrate without the contamination of the substrate ascribable to the sputtering of walls by the plasma.
FIG. 1 is a sectional view, partly in blocks, showing the construction of an example of a prior-art plasma processor disclosed in Japanese Patent Application Laid-open No. 79621/1982. The prior-art plasma processor comprises a plasma generation portion 1. This plasma generation portion 1 has a plasma generating vessel, for example, glass tube 2; radio-frequency electric field formation means, for example, a radio-frequency waveguide 3 which accommodates the plasma generating glass tube 2 and which generates a nonuniform r.f. electric field perpendicular to an axial direction (assumed to be the z-direction); and magnetostatic generation means, for example, a solenoid coil 5 which is arranged around the r.f. waveguide 3 and which is connected to a D.C. power source 4 to generate a nonuniform magnetostatic field in the axial direction. Radio-frequency power is fed to the r.f. waveguide 3 through a magnetron 7 which is mounted on the upper part of the r.f. waveguide 3 and which is connected to a driving power source 6. In addition, a gas, for example, a reaction gas, is fed to the plasma generating glass tube 2 through gas supply pipe 8.
The prior-art plasma processor further comprises a plasma reaction portion 9. In this plasma reaction portion 9, there is disposed a stage 10 on which a substrate 11 to be processed with a plasma is placed. The above mentioned gas supply pipe 8 is connected to the upper part of the plasma reaction portion 9. An exhaust pipe 12 for exhausting the used gas is connected to the lower part of the plasma reaction portion 9.
The prior-art plasma processor is constructed as stated above, and it forms the plasma on the basis of electron cyclotron resonance. .Therefore, this electron cyclotron resonance will be explained below:
Here, B(z) denotes the intensity of the nonuniform magnetostatic field in the axial direction. The r.f. electric power fed into the r.f. waveguide 3 by the magnetron 7 establishes a nonuniform r.f. electric field E.sub.rf (z) within the plasma generation portion 1 which is so shaped as to resonate in accordance with the frequency of the r.f. power. The intensity of the magnetostatic field in the z-direction, which causes the electron cyclotron resonance with the r.f. electric field E.sub.rf (z) within the plasma generation portion 1, lies in a range within the plasma generation portion 1 as shown in FIG. 2. This figure is a distribution diagram showing resonance points in a radial direction r from the center of the plasma generation portion 1 to the side wall surface thereof and in the axial direction z below the top wall surface of the plasma generation portion 1. A curve from point A to point B is obtained by connecting the points of magnetic field intensities at which the magnetostatic field intensity B.sub.z (z) in the z-direction causes the resonance with the r.f. electric field E.sub.rf (z).
Electrons perform a well-known cyclotron motion in the magnetostatic field B, and the angular frequency .omega..sub.c of the cyclotron motion is expressed by .omega..sub.c =eB/m (where e denotes the absolute value of electronic charge, and m denotes the mass of the electron). Letting .omega. denote the angular frequency of the r.f. electric field E.sub.rf (z) in the plasma generation portion 1, when the cyclotron resonance condition of .omega.=.omega..sub.c holds, the energy of the r.f. power is continuously supplied to an electron, and the energy of the electron increases.
Under such cyclotron resonance conditions, a gas of proper gaseous pressure is introduced into the gas supply pipe 8. The electrons generated in a preliminary discharge state are then continuously supplied with energy from the r.f. power. Therefore, the electrons attain a high energy state, and the plasma is developed through the process of collisions. The r.f. power is further supplied to the developed plasma under the resonance conditions.
Accordingly, assuming by way of example that the gas introduced through the gas supply pipe 8 is SiH.sub.4, the r.f. power is properly adjusted in addition to the pressure of the gas, whereby the types, concentrations, and/or energy levels of respective ions such as Si.sup.+, SiH.sup.+, SiH.sub.2.sup.+ and SiH.sub.3.sup.+ can be controlled, and simultaneously, the types, concentrations, and/or energy levels of radicals such as Si.sup.* and SiH.sub.x.sup.* can be controlled.
Meanwhile, an axial force F.sub.z given by the following equation acts on the electron in the presence of both the nonuniform magnetostatic field B(z) and the nonuniform electric field E.sub.rf (z), so that the electrons are accelerated in the axial direction: ##EQU1## where .mu. denotes a magnetic moment, B a magnetic flux density, z a distance in the axial direction, .omega..sub.0 the energy of the circular motion of the electron, B.sub.0 a magnetic flux density in the plasma generation portion 1, m the mass of the electron, and M the mass of the ion.
Accordingly, the electrons in the plasma generated by the plasma generation portion 1 in FIG. 1 are axially accelerated toward the plasma reaction portion 9. In consequence, an electrostatic field E.sub.0 (z) which accelerates the ions is established in the axial direction within the plasma. This electrostatic field E.sub.0 (z) accelerates the plasma as a whole in the axial direction, so that a plasma flow 13 extending in the axial direction appears in the plasma reaction portion 9. Since magnetic lines of forces created by the solenoid coil 5 have components in the r-direction in the plasma reaction portion 9, the plasma flow 13 spreads along the magnetic lines of force.
Such a plasma processor can be applied to various surface processes including plasma etching, plasma CVD and plasma oxidation, and can effectively perform these processes.
With the prior-art plasma processor utilizing electron cyclotron resonance, the intensities of the magnetostatic field B.sub.z (z) and the r.f. electric field E.sub.rf (z) are not uniform in the radial direction. This leads to the problem that, in general, uniformity in the plasma process is difficult to attain. By way of example, when a film is formed by the plasma CVD, the distribution of the thickness of the film becomes nonuniform, especially in the case of a substrate of large diameter, as illustrated in FIG. 3. This figure is a distribution diagram in which the abscissa represents the r-directional distance from the center of the substrate, while the ordinate represents the distribution of the thickness of the film.
Further, magnetic fluxes generated by the solenoid coil 5 flare within the plasma reaction portion 9 as illustrated in FIG. 4, so that the plasma flow 13 flares also. This leads to the problem that the plasma sputters the chamber walls defining the plasma reaction portion 9. Therefore, the substrate 11 might be contaminated with substances flying from the walls. Another problem is that, since the proportion of the plasma reaching the substrate lessens, the efficiency of the plasma process is reduced.